# std::sized_sentinel_for, std::disable_sized_sentinel_for

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class S, class I > concept sized_sentinel_for = std::sentinel_for<S, I> && !std::disable_sized_sentinel_for<std::remove_cv_t<S>, std::remove_cv_t<I>> && requires(const I& i, const S& s) { { s - i } -> std::same_as<std::iter_difference_t<I>>; { i - s } -> std::same_as<std::iter_difference_t<I>>; }; | (1) | (since C++20)
template< class S, class I > inline constexpr bool disable_sized_sentinel_for = false; | (2) | (since C++20)

1) The `sized_sentinel_for` concept specifies that an object of the iterator type `I` and an object of the sentinel type `S` can be subtracted to compute the distance between them in constant time.
2) The `disable_sized_sentinel_for` variable template can be used to prevent iterators and sentinels that can be subtracted but do not actually model `sized_sentinel_for` from satisfying the concept.
A program may specialize `disable_sized_sentinel_for` for cv-unqualified non-array object type `S` and `I`, as long as at least one of which is a program-defined type. Such specializations are usable in constant expressions and have type const bool.

### Semantic requirements

Let i be an iterator of type `I`, and s a sentinel of type `S` such that `[`i`,` s`)` denotes a range. Let n be the smallest number of applications of ++i necessary to make bool(i == s) be true. `I` and `S` model `sized_sentinel_for<S, I>` only if all following conditions are satisfied:

- If n is representable by std::iter_difference_t<I>, then s - i is well-defined and equals n.
- If -n is representable by std::iter_difference_t<I>, then i - s is well-defined and equals -n.

### Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

### Implicit expression variations

A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.

### See also

ranges::sized_range (C++20) | specifies that a range knows its size in constant time (concept)
ranges::size (C++20) | returns an integer equal to the size of a range (customization point object)

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